1. Field of the Invention
This invention relates to an antiskid control device adaptable to a braking system for vehicles.
2. Description of the Prior Art
Supposing that the vehicle speed is V and the circumferential speed of wheel is V.sub.w, then the slip rate S is defined by the following equation: ##EQU1## It is also known that when the coefficient of friction, in the vehicle moving direction, between the wheel and the ground surface contacting therewith is given as .mu., there exists a relation such as depicted by curve a in FIG. 1 between the slip rate S and the coefficient of friction .mu.. In the plotting of FIG. 1, the measure on the vertical axis .mu. is normalized, whereby the maximum value of .mu. is 1. As seen from FIG. 1, generally the coefficient of friction .mu. has the maximum value when the slip rate S is 0.2 or close thereto and lowers with the increase of slip rate S when the wheel is locked against rotation by, for instance, sudden application of brakes. Therefore, in order to let the vehicle stop in a minimum distance by applying a braking force to the wheels, it is desirable that the braking force be applied to the wheels such that the coefficient of friction .mu. will always approach the maximum value during braking, in other words, the slip rate S will be retained close to 0.2. Thus, an antiskid control device for vehicle braking system has been required which is capable of controlling the braking force applied to the wheels so that the slip rate S will take a favorable value as mentioned above.
It is generally difficult to directly measure the slip rate of a running vehicle, for example, an automobile. Therefore, antiskid control devices designed to meet said requirement by using the measurable values relating to the slip rate S have been proposed. In one of such proposed antiskid control devices, the ratio between the rising rate and the lowering rate of the circumferential speed of wheel is simply determined and the coefficient of friction .mu. is estimated therefrom to thereby control the braking force supplied to the wheels. According to such device, however, although a desirable braking performance may be obtained when the vehicle is running on a flat surface and the like, it is unable to produce a satisfactory antiskid action on a rough road since the circumferential speed of wheel won't make smooth increase or decrease and therefore the determined values of said ratio vary widely.